This invention relates generally to systems designed to compensate for the linear distortions introduced by the transmission channels of digital data transmission systems. More particularly, the invention relates to an adaptive phase detection method and apparatus for a digital data transmission system that uses the phase-shift keying (PSK) modulation technique.
The widely used PSK modulation technique is described, for example, in books entitled "Data Transmission", by R. W. Bennett and J. R. Davey, Chapter 10, McGraw-Hill, New York, 1965, and "Principles of Data Communication", by R. W. Lucky, J. Salz and E. J. Weldon, Jr., Chapter 3, McGraw-Hill, New York, 1968. In the PSK modulation technique, the sequence of bits to be transmitted is first converted by groups of 2, 3 or 4 bits, into a sequence of symbols. The number of different symbols is generally equal to two raised to the power of the number of bits in a group. This sequence of symbols is then transmitted, one symbol at a time during periods which have a T-second spacing and which have a predtermined phase at specific points in the period called signalling instants. Each symbol may take the form of a pulse modulated by a carrier signal whose phase for each symbol exhibits a corresponding phase change relative to the phase of the immediately preceding symbol. These modulated pulse symbols are fed to a transmission channel whose output is connected to a data receiver. The function of the transmission channel is to provide at the receiver a signal relatively similar to the input signal applied thereto. The receiver examines the signal received from the transmission channel at each signaling instant to determine its phase and the corresponding transmitted data. In actual practice, mainly for reasons of cost, the telephone lines of the public network are most commonly used as transmission channels. However, telephone lines, while satisfactory for voice transmission purposes, are not quite adequate to transmit data pulses at a relatively high data rate with a very low probability of error. On any telephone line of a give quality, there will be amplitude and phase distortions that will alter the shape of the pulses being transmitted. At pulse transmission rates above some minimum rate, these distortions will create, at the signaling instants, an interaction between successive pulses, which makes it difficult for the receiver to correctly detect the data in the signal. This interaction is known as intersymbol interference. To compensate for the effects of the intersymbol interference, the receiver is usually provided with a device called an equalizer. One of the more widely used type of equalizer is the so-called automatic adaptive equalizer which has been discussed in many publications and is described, for example, in Chapter 6 of the book by R. W. Lucky et al. mentioned earlier. An equalizer generally consists of a network whose transfer function is adjusted to meet a given performance criterion. Such an equalizer is a complicated device which in a digital form requires a very high computing power and is, consequently, expensive. The incorporation of a digital equalizer in a receiver will, therefore, considerably increase the cost of the latter. The decision as to the type of equalizer which should be incorporated in a receiver will, of course, depend not only on the performance level required to enable the receiver to operate satisfactorily, but will also depend on the cost advantage to be gained thereby. An equalizer must be used in certain cases, for example, where the transmission rate is equal to or higher than 4800 bits per second (bps). On the other hand, an equalizer will not normally be required if, for example, the transmission rate is lower than 2400 bps or if the quality of the transmission line is very high. However, the use of an equalizer which would be unnecessary may be desirable in some cases, for example, in a receiver designed to accommodate a great many different lines some of which might have marginal characteristics in relation to accepted standards. In these cases, it is apparent that there is a need for an inexpensive device capable of compensating for the effects of the intersymbol interference without however requiring the very high computing power associated with an equalizer for high data rates. Accordingly, the present invention is designed to improve the phase detection system by the use of an apparatus which will compensate for the effects of the intersymbol interference.